It is a slang word that came about from people saying "chill out and relax". People use the word chillax all the time these days. People between the ages of 13 to 39 use this word as the word of choice when they want to sound cool. Usually when someone is acting out or being crazy. There is a drink called Chillax drink. It is a 2oz shot that you can drink to "Chillax" It works to help you focus, relax and understand what is going on better around you.